Refractory mold having high permeability, composition for making, and method of making same



Nbv. 25, 1958 H. L. BENHAM REFRACTORY MOLD HAVING HI 2 GH PERMEABILITY, COMPOSiTION FOR MAKING AND METHOD 'OF MAKING SAME Filed Feb. 1, 1957 INVENTOR.

A T TOR/v5 Y smoothness.

2,861,308 7 REFRACTORY MOLD HAVING HIGH PERME- ABILITY, COMPOSITION FOR MAKING, AND METHOD OF MAKING SAME Harold L. Benham, Bedford, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application February 1, 1957, Serial No. 637,768 10 Claims. (Cl. 22-193) This invention relates to a highly permeable'refractory mold for casting metal and particularly to an improved investment material for forming such a refractory mold.

In precision casting metallic articles by means of refractory molds it is frequently necessary that the casting surfaces of the molds Thus, in investment molding it is usually desirable to form the principal refractory portion of the mold of a hard material to which a coating. on the invested pattern will tightly adhere, thereby providing the mold with a smooth casting surface upon removal of the destructible pattern. The mold must have sufficient strength and rigidity so as towithstand the' pressure of the molten casting metal. It is important, moreover, to use an investment material which produces a mold also having proper porosity, and the surface and setting characteristics of the investment mixshouldpermit it to bond tightly to the coating on the pattern.

Accordingly, a principal object of the present invention is to provide an improved investment composition for forming highly permeable refractory molds tobe used in precision castingoperations. A further object of this invention-isto providean inexpensive investment mold and a:process.for forming the. same by means ofianinvestment material which produces more usablemold-per weight of material than those heretofore usually employed.

These and other objects are attained in accordance with this invention with an investmentmaterial consisting es seutially of.-a.mixturet of a dry mix or grog-, a small amount ofstearic acid, and an ethylsilicate binder. The investment grog contains a pulverized fire clay, afine refractory fillermaterial, and an accelerator for the binder. Borax glass-is preferably includedinthe mixture to increase the high temperature bond of the mold. Upon firing, this investmentmaterial has excellent permeability as well as being strong and rigid;

An investment mixformed'of the aforementioned type of grog and ethyl silicate. binder is disclosed and claimed in.copending patent:application Serial No. 324,706, filed on December 8, 1952, now-abandoned, in the names of James P. Bradley and Robert R. Dohrmann and'owned by. the assignee of the present invention. Refractory molds-formed from this mix have proved to be satisfactory for casting many types of high temperature alloy turbine buckets. However, when subjected to vibration afterbeing poured around. the destructible pattern, the investinent material settles and becomes a dense mass which isi'not very permeable. On the-other hand, when a small amount of stearic acid is included in the investment material in accordance with the present invention, the resultant mold-is considerablymore permeable,-thereby permitting gases-and vapors which are formed during pouring of the molten;casting metal to more'readily escape. Moreover, the; new investment mix provides approximately 25% more-,usable-moldper weight of 'mix because itprevents settling,of comminuted refractory constituents in the investment-grog. It also has been fonndthat these highly permeable refractory molds are much more easily knocked possess high strength and exceptional United States Patent 2,861,308 Patented Nov. 25,

2 out after the castings have solidified than those heretofore employed. Furthermore, the investment material described herein is more versatile and of broader utility since differences between the expansion rate of the dip coat and the expansion rate of the secondary investment material during the firing operations are less critical than formerly. I

Other objects and advantages of this invention will more fully appear from the following detailed description thereof in conjunction with the accompanying drawing, which contains a somewhat schematic sectional view of a destructible pattern invested in a refractorymol'd formed in accordance with the invention.

Referring more particularly to the drawing, a pattern It) of a turbine bucket to be cast is shown invested in a refractory mold 12 within a metallic container or flask 14 positioned on a base plate 22. The pattern 10 is preferably formed of a low fusing substance, suchas waxror a thermoplastic material, or any other vaporizable, fusible, combustible or otherwise readily destructible material. Plastic patterns are frequently employed in order to'p'rovide optimum results for many applications; Among the plastic patterns which have been found to be satisfactory are those formed of polystyrene, although other thermoplastic pattern materials, such as resinous polymerized derivatives of acrylic acid and resinous polymerized derivatives of methacrylic acid, may be used.

The pattern may be formed in a conventional manner by injection molding. A gating portion 16 having a pouring basin part 18 at its outer endis shown as" attached to the pattern, the portions 16 and 18 usually being of'a destructible material similar to that'of the pattern. After the pattern is molded, it is preferably cleaned with an alcohol solution and air dried prior to application of-the coating solution. Before the pattern is invested within the mold, it is coated with an appropriate coatingmaterial'which is to form thecasting surface 20 OftheFre'fractory mold. This coating material may comprise" an aqueous dispersion of conventional finely comminuted refractory materials, a binder such as-an air-setting silicate cement, and defoaming and wetting agents. Gelatineand acids can also be included in-the coating-mixture. This type of coatingis disclosed in United States Letters Patent No. 2,752,257, which issued on June26, 1956, in the names of James P. Bradley and RobertR. Doh'rmann.

Coating of the pattern is preferably accomplished by dipping the pattern in the coating solution. Although in some instances the coating may also be applie'd'by spraying or painting it on the pattern or in any other suitable manner, dipping is preferred because it assures more uniform coating of all the pattern surfaces and 'is the simplest method of application. 7

The dip coat slurry is preferably kept in constant motion by stirring means except during-the actualdipping operation. However, the mixing action should not be such as to unnecessarily introduce air into" the slurry. The destructible pattern used for forming turbine buckets or other precision cast parts is immersed in the "dip'coat slurry, preferably to Within A2 to %1. inches of the end of the gating portion of the pattern. Care should'be exercised in immersing the pattern in the slurry to prevent air entrapment on the pattern. Normally the dip coat solution is retained at room temperature during the dipping operation because excessive heat can result in distortion of the plastic pattern. The surplus coating material is permitted to drain off prior to subsequent treatment and investment. 7

After the pattern has beencompletely coated with the dip coat slurry, it maybe sanded or stuccoed; to vprovide a rough surface on the coating thus insuring greater adhesion between the principal refractory portion 12 of the mold and the dip coat 20 on the pattern. This sand- 3 ing may be accomplished by merely screening or otherwise applying silica or other suitable refractory materials in known manner to the outer coated surface of the destructible pattern. When all the molding surfaces of the pattern have been effectively covered with sand, the

pattern should be air dried.

the following procedure provides excellent results. The

base plate 22 is preferably first sprayed or otherwise coated with molten wax so as to form a thin film of wax over its upper surface 24. Before the wax is completely solidified, the pattern to be invested is positioned on the plate 22 with the gate and pouring basin portions 16 and 18 thereof extending downwardly and seated firmly in thewax film. The sleeve or flask 14 is next placed around the pattern and pressed lightly into the wax layer. In order to completely seal the flask 14 to the plate 22, it is preferable to again spray or pour molten wax over the outer surfaces of these parts at their junction 26. The wax in the resultant assembly should be allowed to thoroughly solidify "before proceeding further.

After the refractory mixture, which will be hereinafter described in detail, has been mixed with a proper amount of the liquid binder, it is poured into the sleeve or flask 14. The flask is preferably vibrated during this pouring operation, and the mold is then allowed to set.

When the mold body has solidified or set to a sufficient extent, the base plate 22 is removed from beneath the mold and heat is applied to melt the pattern. It is necessary to apply sufficient heat to raise the mold temperature above the fusing point of the material, thus permitting the molten pattern to escape through the gate and sprue opening in the mold formed by the pattern portions 16 and 18. In this manner the dip coat which had covered the pattern tightly adheres to theremainder of the mold and provides the casting cavity with a smooth coating. It is also possible to vaporize the pattern, if a vaporizable material is used, by heating the mold rapidly to a high temperature.

After removal of the pattern from the mold in the foregoing manner, the mold is fired or burned out to remove substantially all the volatile matter. The mold is then preferably preheated to the desired temperature, and the molten casting metal is poured or otherwise introduced into the mold cavity formed by the pattern. In the majority of instances it is necessary to pour the casting metal while the mold is still hot in order to obtain satisfactory results. After the molten metal has been poured and the casting has solidified, the refractory mold body 12 and the adhering coating 20 may be broken to permit the removal of the casting. The finished casting possesses excellent surface smoothness and detail and requires little finishing.

The investment material used to form the body or principal refractory portion 12 of the mold in accordance with the present invention consists of a dry mix or grog to which is added a small amount of stearic acid and an ethyl silicate binder. This dry mix comprises major proportions of a pulverized fire clay, such as dead burned fire clay or fire brick, and a finely comminuted refractory material and a minor proportion of an accelerator or gelation agent for the'binder. Magnesium oxide is the setting accelerator preferably employed, but magnesium carbonate, calcium carbonate, sodium carbonate, and other alkaline oxides or carbonates may be used. In order to improve the high-temperature bond of the resultant mold,

-borax glass is preferably also includedin the mix. The

binder for the grog consists of an aqueous solution of condensed ethyl silicate, alcohol and an acid. Denatured ethanol of approximately 190 proof spirit is the alcohol preferably employed, while concentrated hydrochloric acid is the acid which I have found most desirable to add to the binder solution.

Accordingly, a satisfactory investment dry mix or grog is one comprising, by weight, approximately 65% to 90% of the finely ground burned fire clay or brick, 9% to 34% silica fiour or other finely comminuted refractory material, and 0.15% to 1.5% of an accelerator or gelation agent, such as magnesium oxide, for the binder. In order to provide optimum results, the fineness of this mix should be between about 90 and 100 A. F. A. An accelerator content of approximately 0.2% to 1% is preferred for most applications, the greater the amount of accelerator added, the lesser the gel time. When borax glass is included in the grog to provide the resultant mold with improved high-temperature bond, it should be present in an amount not in excess of about 2% by weight. Moreover, quantities of borax glass as small as 0.1% have proved to be beneficial in improving the high-temperature bond of the mix. At present we prefer to use a dry mix containing about 0.3% to 1% borax glass. H

After the dry mix or grog has been prepared, a small amount of water is added to it and the resultant mixture mulled for a short period of time. A mulling period of approximately 30 seconds to 1 /2 minutes has proved to be satisfactory. Next the stearic acid is added, about 400 grams of stearic acid to 60 pounds of grog being preferred for many applications. The exact amount of stearic acid to be used depends to a considerable extent upon the density and permeability of the refractory mold it is desired to form. Generally the stearic acid should constitute about 0.5% to 3% of the total weight of the grog. Approximately 50 to cc. of water is all that is necessary for most applications, this amount being equivalent to about 0.2% to 0.6% of the weight of the grog. It is desirable to mull the mix for about two to five minutes, depending on the size of the batch, after the stearic acid has been added.

The inclusion of stearic acid in the investment mixture causes the liquid in the final mix to suspend the refractory particles of the grog. As a result, the particle mobility in the investment mix is practically zero, and when this mix is subsequently vibrated, the grog materials do not settle. In turn, the final mold after burn-out possesses exceptionally high permeability. The exact mechanism which causes this suspension action is not known. It is believed that the small amount of water which is added to the dry grog serves to ionize a portion of the magnesium oxide, which then reacts slightly with the stearic acid to form magnesium stearate. However, the direct addition of magnesium stearate as a substitute for the stearic acid and water does not provide the same results. While a. mold formed in accordance with the present invention is highly permeable, a mold produced by an investment composition containing magnesium stearate rather than stearic acid has a permeability which is not appreciably different from that of conventional investment molds of this type.

The aforementioned ethyl silicate type of binder preferably comprises, by weight, approximately 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5% to 13% water. Depending on the particular application, the ratio, on a weight basis, of the dry mix or grog to the ethyl silicate binder may vary from about 2.5 to 4.5. When the above investment dry mix and the binder are mixed in the proper ratios, the resultant investment material generally comprises, by weight, approximately 43% to 74% pulverized fire clay or brick, 6% to 25% silica flour, 0.2% to 1.8% stearic acid, 0.1% to 1.2% magnesium oxide, 6% to 17% condensed ethyl silicate, 6% to 17% alcohol, 0.02% to 0.1% hydrochloric acid to 3.7% water. A sufiicient amount of additional water may be added during mixing of the grog and; binder to raise the water contentto ashigh as: 7%. lffborax glass is included in the mixture. it preferably constitutes between 0.1% and 126% of the final investment material; For optimum results the burned fire clay content should be between 55% and 70% of the weight of the entire mixture, this amount being equivalent to approximately 70% to 87% of the dry mix or grog.

The finely ground fire clay functions as the refractory base of the investment mixture and must be selected so that it has desirable expansion characteristics. This material should also be of uniform quality and composition and reasonably free of foreign matter. Calm o" is an example of a pulverized dead burned fire clay. Various mixtures of relatively coarse and fine Calmo fire clays may be used, these clays varying in A. F. A. finenesses from approximately 20 to 120.

Silica flour is added to increase the lines in the refractory base and to eliminate voids in the backing of the mold. Other refractory powders, such as zirconium silicate or zirconium oxide flour, may be used for particular applications, however. Generally itis desirable to use a silica flour which is fine enough to permit at least 99% of it to pass through a 140 mesh screen. The magnesium oxide is used in powdered. form and may consist of either heavy or light magnesium oxide, or mixtures of these oxides. It is preferable that this material be of sufiicient fineness so that at least 90% of it will pass through a 200 mesh screen. Likewise, the borax glass, which functions as a secondary high-temperature bonding agent in the investment mix, should be added in powder form, preferably of approximately 90 to 110 mesh.

The condensed ethyl silicate in the investment binder, of course, is a source of silica for the reaction in which ethyl silicate and water react to form silica gel and alcohol. Upon drying, silica is the ultimate binder for the investment. Alcohol is included to produce mutual solubility of ethyl silicate and water since these latter constituents are immiscible in the absence of the alcohol. The hydrochloric acid is necessary for pH control and to regulate the speed of the aforementioned reaction and the subsequent mold gelation.

It is preferable to use a condensed ethyl silicate having not less than 25% available silica as SiO An example of such an ethyl silicate is one consisting of approximately 85% tetraethyl orthosilicate and polysilicates. Satisfactory commercially available products of this composition frequently have 0.1% maximum acidity as hydrochloric acid, a flash point (open cup) of approximately 90 F., and a specific gravity between 0.920 and 0.950 at C. Hence a typical example of the ethyl silicate solution preferably used is a condensed ethyl silicate which contains, by volume, approximately 50% ethyl silicate, 0.1% hydrochloric acid, and the balance alcohol and water. It will be appreciated, however, that any hydrolyzed or condensed ethyl silicate solution, may be satisfactorily used so long as the ethyl silicate content is sufiicient to provide the proper bonding properties.

With respect to the permissible upper limit of the concentration of the ethyl silicate solution, it is desirable that condensing conditions exist. Accordingly, to assure these conditions, I have found it feasible to use a solution which contains only enough water to provide a sufiicient amount of hydrolysis to obtain the above satisfactory bonding and strengthening effects, or to otherwise use ethyl silicate under hydrolyzing conditions. Thus, I have obtained best results by the use of a mixture containing hydrolyzed or condensed ethyl silicate solution with the ethyl silicate content between and 75% by volume.

In preparing the binder solution, the water and acid are first mixed together, and the alcohol and condensed ethyl silicate are subsequently added. The solution is then stirred and permitted to set for several hours prior to use. A preferred method of mixing the grog and liquid bindenconsistsof placing the dry grog, together with secondary'water equal to about 2% of the weight of the binder, in a rotating batchmixer. The stearic acid is thenadded, andthe mixture is mulled for approximately three minutes. Next the binder solution is introduced and the resultant slurry mixed for about ten minutes. Although it is preferable to follow the above-described procedure, in some instances satisfactory results can be obtained by first mixing the dry grog and binder solution and thereafter adding the stearic acid;

The resultant highly permeable refractory mold for investment molding is particularly adapted for use in casting articles having curved surfaces, such as turbine buckets, because it tightly adheres to the coating on the destructible patterns and prevents distortion of this coating. Furthermore, the resultant mold does not react with nickel base alloys, a material used in cast turbine buckets, and hence has no adverse effect on the surface qualities of such buckets.

While the present invention has been described by means of certain specific examples, it is to be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

I claim:

1'. In a refractory mold composition consisting predominantly of co-mminuted refractory material and a silicatetype hinder, the improvement which consists of including therewith stearic acid in an amount sufiicient to cause said refractory material to be suspended in said binder and thereby produce a highly permeable mold.

2. A mold composition consisting essentially of a mixture of an ethyl silicate solution and a grog consisting essentially, by weight, of approximately 65% to of a pulverized fire clay, 9% to 34% of a finely comminuted refractory material, 0.5% to 3% stearic acid, and 0.15% to 1.5% of a setting accelerator for the ethyl silicate solution.

3. A highly permeable refractory mold having a body portion resulting from setting of a mixture of an ethyl silicate solution and a grog consisting essentially, by weight, of approximately 65 to 90% of a finely pulverized fire clay, 9% to 34% silica flour, 0.5% to 3% stearic acid, and 0.2% to 1% of a setting accelerator for ethyl silicate, the ratio, by weight, of the grog to the ethyl silicate solution being between 2.5 to 1 and 4.5 to 1.

4. A mold composition consisting essentially of a mixture of a grog consisting essentially, by weight, of approximately 65% to 90% of a pulverized burned fire clay, 9% to 34% of a finely comminuted refractory material, 0.5% to 3% stearic acid, 0.15% to 1.5% of a setting accelerator for ethyl silicate, and an ethyl silicate binder solution consisting essentially, by weight, of about 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% of an acid, and 5% to 13% water.

5. An investment composition for a highly permeable refractory mold consisting essentially of a mixture consisting essentially, by weight, of approximately 70% to 87% of a finely ground dead burned fire clay, 9% to 34% silica flour, 0.5% to 3% stearic acid, and 0.2% to 1% magnesium oxide and an ethyl silicate binder solution consisting essentially, by weight, of about 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5% to 13% water,

-6. A refractory mold characterized by high permeability, said mold having an investment body portion formed of the residue of a mixture consisting, by weight, essentially of approximately 43% to 74% of a pulverized fire clay, 6% to 25% of a finely comminuted refractory material, 0.2% to 1.8% stearic acid, 6% to 17% condensed ethyl silicate, 6% to 17% alcohol, 0.02% to 0.1% of an acid, 0.9% to 7% water, and 0.1% to 1.2% of a setting accelerator for the condensed ethyl silicate.

refractory mold consisting essentially,

7. An investment composition for a highly permeable by weighgof ap proximately 55% to 70% of a finely pulverized dead burned fire clay,'6% to 25% silicaflour, 0.2% to 1.8% stearic acid, 0.1% to 1.2% magnesium oxide, 0.1% to 2% powdered borax glass, 6% to 17% denatured ethyl alcohol, 0.02% to 0.1% concentrated hydrochloric acid, 0.9% to 3.7% water, and 6% to 17% condensed ethyl silicate solution, the ethyl silicate content in said solution being between 25% and 75% by volume.

8. A method of forming a refractory mold which comprises coating a readily destructible pattern with a re- -fractory coating mixture, investing said coated pattern in a refractory molding mix consisting essentially of a grog consisting essentially, by weight, of' approximately 65% to 90% of a finely ground burned fire clay, 9% to 34% silica flour, 0.5% to 3% stearic acid, and 0.15% to 1.5% of a setting accelerator for ethyl silicate, and a binder for said grog consisting essentially of a solution of approximately 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% of an acid and 5% to 13% water, and thereafter eliminating the pattern from said mold, whereby said coating adheres to the refractory mold.

9. The process of forming a highly permeable refractory mold having a smooth casting surface, said process comprising applying a refractory coating to a fusible pattern, drying said coating, thereafter investing said coated pattern in an investment material consisting essentially, by weight, of approximately 43% to 74% of a finely pulverized dead burned fire brick, 6% to 25% silica flour, 0.2% to 1.8% stearic acid, 0.1% to 1.2% magnesium oxide, a smallv but effective amount of borax glassnot in excess of 2%, 6% to 17% condensed ethyl silicate, 6% to 17% ethyl alcohol, 0.02% to 0.1% concentrated hydrochloric acid and 0.9% to 7% water, setting the investment material, melting and removing the pattern from the formed mold, whereby the coating tightly adheres to the walls of the casting cavity of the investment material, and thereafter heating said mold at a temperature suflicient to remove substantially all of the volatile matter therefrom.

10. A precision casting mold having a body of refractory material resulting from setting of a mixture of a silicate-type binder solution and a grog consisting essentially of 0.5% to 3% by weight of stearic acid and the balance substantially all comminuted refractory material, the ratio, by weight, of the grog to the silicate-type binder solution being between 2.5 to 1 and 4.5 to 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,521,839 Feagin Sept. 12, 1950 FOREIGN PATENTS 725,319 Great Britain Mar. 2, 1955 

10. A PRECISION CASTING MOLD HAVING A BODY OF REFRACTORY MATERIAL RESULTING FROM SETTING OF A MIXTURE OF A SILICATE-TYPE BINDER SOLUTION ANDA GROG CONSISTING ESSENTIALLY OF 0.5% TO 3% BY WEIGHT OF STEARIC ACID AND THE BALANCE SUBSTANTIALLY ALL COMMINUTED REFRACTORY MATERIAL, THE RATIO, BY WEIGHT, OF THE GROG TO GHE SILICATE-TYPE BINDER SOLUTION BEING BETWEEN 2.5 TO 1 AND 4.5 TO
 1. 